1. Field of the Invention
This invention relates to a method for controlling a wire cut electrical discharge machine tool in which a workpiece is cut along an outline. More specifically, it relates to a machining control method which makes it possible to accurately machine corners in the outline, without producing detours in the machining outline.
2. Description of the Prior Art
In general, a wire cut electrical discharge machine tool has a worktable which supports the workpiece and a wire electrode guide arranged vertically above and below the workpiece. In a wire cut electrical discharge machine tool, either the work table or the wire electrode guide (or both) moves horizontally along the X-axis and the Y-axis to cut the workpiece along a predetermined outline. The motion of the worktable or wire electrode guide along the X-axis or Y-axis is controlled by a suitable control device such as a computer or numerical control device. The outline along which the workpiece is to be cut and various machining conditions are programmed prior to machining and stored in the control device memory.
In a wire cut electrical discharge machine tool with the said component parts, it is known that when wire cut electrical discharge machining is performed the reaction force due to an electrical discharge causes the machining sections of the wire electrodes to bend in the direction opposite to that in which the machining is proceeding. That is to say, in a wire cut electrical discharge machine tool, when an electrical discharge occurs it is difficult to avoid being bending of the wire electrodes due to the reaction force. Even if the wire guide follows the outline accurately, the actual position of the wire electrodes during machining lags in the direction in which the machining is proceeding.
Consequently, when a corner in the outline is machined, the wire electrode follows the guide along a detoured inner path as in the relation between the front and rear wheels of an automobile. Deviation from the accurate outline is produced at the corners of the outline which are machined as rounded rather than sharp corners, reducing the machining accuracy.
In addition, when machining is being done along a straight line section of the outline, since the areas which are machined per unit time on the left and right sides of the centers of the wire electrodes are equal, the wire electrodes are bent only on the side that is retarded in the direction of advance of the wire electrodes. However, at an angular change of direction point, when the direction in which the machining is proceeding is changed, the right and lift machined areas in front of the advance of the wire electrodes become unequal, so that the wire electrodes are bent toward the direction in which less machining is taking place. Wire electrode bending of this type also contributes toward lowering the machining accuracy at angular corners of the outline.
In order to solve the problem described above and reduce the amount of wire electrode bending, the tension on the wire electrodes has been increased as much as possible, and the distance between the upper and lower wire electrode guides has been made as short as possible. By such means the amount of wire electrode bending can indeed be reduced. However, such means cannot reduce the amount of wire bending to zero and therefore are not really satisfactory.